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When I enabled GMC by default, I accidentally enabled it for ChromeOS
(again, see crrev.com/c/1761779). This CL makes GMC only enabled for
Windows, Mac, and Linux.

Bug: 1045550
Change-Id: I6ae57cae0569c05a03da0c1f15763d97dc1b5f85
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2023045Reviewed-by: Mounir Lamouri <mlamouri@chromium.org>
Commit-Queue: Tommy Steimel <steimel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#735873}